Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0a1) Gecko/20120212 Firefox/13.0a1
STR (requires dual monitor setup):
1. Load http://pearce.org.nz/full-screen/
2. Press 'f' key to enter fullscreen.
3. Click on the desktop in your other monitor, fullscreen will exit.
4. Focus the browser window again and press 'f', a fullscreenerror will be thrown.
In the webconsole the following error is logged:
"Request for full-screen was denied because requesting element is not in the currently focused tab."
Problem doesn't occur in FF10.0.1 [Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.1) Gecko/20100101 Firefox/10.0.1], though I can't focus the chrome UI after following the STR above (possibly related to 718939?).
Most likely a regression from bug 706672, which merged to m-c in that range.
This should be fixed by bug 706672.